Sequences which are homologous to the p21 transforming genes of Harvey and Kirsten murine scarcoma viruses have been found in normal human cellular DNA and other vertebrate DNAs. The human genome contains at least four different genes which are homologous to p21 genes. At least one of these normal human genes can, when expressed at high levels, induce tumorgenic transformation of mammalian cells. The transforming genes of several human tumors have been shown to be p21 coding genes. The molecular genetics of the transforming function of the Bovine Papillomavirus has been further defined. The transforming gene is contained within a 2.3 kb segment of viral DNA. Long term interferon treatment of Bovine Papillomavirus transformed cells results in a decrease of viral DNA, loss of transformation, and curing of some cells of the viral genome. The importance of MCF viruses in the pathogenesis of murine thymic tumors has also been studied. Endogenous MCF-like viral DNAs have been found in all strains of Mus musculus. In ecotropic virus induced tumors, recombinant MCF viruses are found in each tumor. At least two different recombinational events between different viral genomes have occured for the formation of oncogenic MCF viruses.